U-shaped magnetic circuit including three permanent magnets separated by pole pieces



Sept. 3, 1968 o. JQHARRA 3,400,349

U-SIIAPED MAGNETIC CIRCUIT INCLUDING THREE PERMANENT MAGNETS SEPARATEDBY POLE PIECES Filed Jan. 14, 1966 FIG I FIG.2

50 I6 so I K Va? 44 x /t x u INVENTOR.

' DAVD J. HARR'A BY M AIR GAP POSITION NEY United States Patent 01 fice3,400,349. Patented Sept. 3, 1968 3,400,349 U-SHAPED MAGNETIC CIRCUITINCLUDING THREE PERMANENT MAGNETS SEPARATED BY POLE PIECES David J.Harra, Menlo Park, Calif., assignor to Varian Associates, Palo Alto,Calif., a corporation of California Filed Jan. 14, 1966, Ser. No.520,606 6 Claims. (Cl. 335-210) ABSTRACT OF THE DISCLOSURE The web orconnecting portion of the U is for-med by a single bar magnet while eachof the arms of the U, perpendicular to the web, includes a bar magnetpoled in magnetically aiding relationship to the magnet in the web. Apair of pole pieces magnetically interconnect the web and arms of the U,while another pair of pole pieces at the ends of the arms are tapered toprovide a region of uniform flux density between the arms.

The present invention relates generally to U-shaped magnetic circuits.More specifically, it appertains to a magnetic circuit including aplurality of straight permanent magnets separated by pole piecesarranged in a U- shaped configuration which is particularly suited foruse with ionic vacuum pumps.

U-shaped magnetic circuits fall into two general categories; thosehaving curved or C-type magnets, and those having bar-type magnetsforming straight segments of a rectangular U-shaped magnetic circuit.Both known C- type magnets and rectangular U-shaped magnetic circuitsare characterized by various limitations and disadvantgaes. For example,C-type magnets have varying magnetic lengths measured along thedirection of magnetization. As is well known, the maximum magnetic fieldproduced in the air gap of a given magnet occurs when the magnetmaterial is operated at the knee of the demagnetization curve. For agiven air gap, the point of operation on the demagnetization curve willvary as the magnetic length. Hence, in order for all of the magneticmaterial to be operating at the knee of the demagnetization curve, thelength of the magnetic material along the direction of magnetizationmust be uniform. Since the C-type magnets have varying magnetic lengths,its operating efliciency, i.e., maximum air gap field intensitythroughout a given air gap size for a given magnet size, is inferior incomparison to a magnet characterized by a uniform magnetic length.Furthermore, the manufacturing process of such C-type or otherirregularly shaped permanent magnets is exceedingly more complex thanthe process employed in manufacturing bar-type magnets.

To overcome the problems characterizing the C-type magnets, variousU-shaped magnetic circuit configurations employing straight bar-typemagnets and pole pieces have been used. One such type employs a singlebar magnet having a pole piece extending perpendicularly away from eachmagnetic pole. Such magnetic circuits are capable of generating air gapmagnetic field intensities comparable to the coercive force coefficientof the magnets employed. Hence, such magnetic circuits can optimallyproduce magnetic fields of, essentially, only a given intensity,therefore being quite inflexible.

To be able to provide magnetic fields of various intensities with amagnetic circuit employing magnets of a given type material while alsopreventing large flux leakages, magnets are placed close to the air gap,generally, in the arms of the U-shaped configuration. Such a magneticcircuit includes a pole piece having a bar magnet extendingperpendicularly away from each end of the pole piece. The latter type isdescribed in the United States Patent 3,182,234 to Paul Meyerer,entitled Permanent Magnet System for Focusing an Electron Beam in aTravelling Wave Tube, issued May 4, 1965, particularly, the embodimentof FIGS. 1 and 2. However, in each of these cases, the structure of themagnetic circuit required to generate a given air gap magnetic fieldintensity is large, and in the case of the Meyerer type structure, theprofile of the magnetic field intensity established in the air gap ispoor, i.e., far from being uniform.

Considerable advantage is therefore to be gained by the provision of amagnetic circuit configuration which overcomes those limitations anddisadvantages characterizing the prior art configurations. Otheradvantages will be realized Where a compact and efiicient magneticcircuit characterized by a more uniform air gap magnetic field intensityprofile is employed to guide the ionizing electrons of a sputter-ionvacuum pump having a cellular anode structure.

The present invention provides an improved U-shaped magnetic circuitconfiguration which is characterized by being compact and efficient,having a uniform magnetic length, being capable of generating with agiven magnetic material an air gap magnetic field of any desiredmagnitude of intensity, and having an enhanced air gap magnetic fieldintensity profile by utilizing what is commonly considered lost fluxleakage to increase the air gap magnetic field intensity. Morespecifically, the magnetic circuit configuration of the presentinvention includes a first straight permanent magnet having a pole pieceat each of its poles. Second and third straight permanent magnets aredisposed in magnetic potential adding relation with the first magnet.The second magnet is disposed to extend from the pole piece at one poleof the first magnet in a direction perpendicular to the direction ofmagnetization of the first magnet. Similarly, the third magnet isdisposed to extend from the other pole piece in direction perpendicularto the direction of magnetization of the first magnet. Additional polepieces as required are disposed to extend from and in the same directionas the pole ends of the second and third magnets distal the first magnetto complete the ,U-shaped magnetic circuit configuration.

By constructing the magnetic circuit with straight magnet sections whichare separated by pole pieces so that magnetic paths are straight, themagnetic length of the circuit is made uniform, hence providing a moreefficient magnetic circuit. Furthermore, by utilizing a magnet in eacharm and at the web section of the U-shaped configuration, an enhancedair gap magnetic field intensity profile is provided as a result of thenormally considered lost flux leakage of the web disposed magnetpermeating the air gap adding with the field generated by the armdisposed magnets. Also, such magnet placement facilitates varying themagnitude of the air gap magnetic field intensity. Moreover, the threepiece magnet structure allows for the construction of a magnetic circuitrequiring less material than prior art circuit configurations for agiven air gap and magnetic field intensity. Consequently, the efiiciencyof the magnetic circuit of the present invention will be improvedconsiderably over those of the prior art.

Accordingly, it is an object of the present invention to provide aflexible U-shaped magnetic circuit having an enhanced efiiciencycharacteristic.

More particularly, it is an object of the present invention to provide aU-shaped magnetic circuit having a uniform magnetic length and whichgenerates an enhanced air gap magnetic field intensity profile.

Another object of the present invention is to provide a compact U-shapedmagnetic circuit including magnets constructed of a given material whoseair gap magnetic field intensity magnitude may be varied as desired.

A further object of the present invention is the provision of a compactU-shaped magnetic circuit configuraof FIG. 1.

FIG. 3 is a top view of a two element unit sputter-ion vacuum pump andmagnets.

FIG. 4 is a graph representing the air gap magnetic field profile of themagnetic circuit of the present invention as compared with the prior artsystems with curve (a) depicting the profile of the magnetic circuit ofthe present invention, curve (b) depicting the profile of a Meyerer typemagnetic circuit, and curve (c) depicting the profile of a U-shapedmagnetic circuit employing a single magnet disposed at the web of the U.

Referring to FIGS. 1 and 2 the magnetic circuit 11 comprises threestraight magnets 12, 13 and 14 arranged in a magnetic potential addingU-shapw configuration to define an air gap 15. The magnets are disposedwith magnet 12 defining the web and each of the magnets 13 and 14defining a segment of one of the arms of the U- shaped configuration.Although the magnets may be of any cross sectional configuration, forsake of simplicity, a rectangular or square configuration isrecommended.

To provide a magnetic circuit 11 having a uniform magnetic length, thenorth pole 16 of magnet 12 is coupled to the south pole 17 of sidemagnet 13 by a rectangular corner pole piece 18 interposed therebetween.Since pole pieces are constructed of low reluctance material and hencedo not provide any magnetization, any pole piece configuration can beemployed without affecting the mag netic length characteristics of themagnetic circuit 11. However, a more efiicient magnetic circuit which isconsiderably easier to manufacture is obtained by employing rectangularblock pole pieces whose sides adjacent the .poles of the magnets are ofthe same cross sectional area and geometric configuration. Hence, in thepreferred arrangement of the magnetic circuit 11, perpendicularlyextending sides 19 and 21 of pole piece 18 are mated respectively withthe north pole 16 of magnet 11 and south pole 17 of magnet 13.Similarly, the south pole 22 of magnet -12 is coupled by a secondrectangular corner pole piece 23 to the north pole 24 of a second sidemagnet 14.

In many magnetic circuit applications, e.g., for establishing a magneticfield to trap and guide the ionizing electrons in ionic vacuum pumps, itis desirable to provide a nearly uniform magnetic field profile in theregion where the pump elements are disposed in order to minimize theestablishment of pressure gradients due to an uneven concentration ofelectrons. Furthermore, it is desirable to provide such a nearly uniformmagnetic field profile over a large area. In such cases, it iscontemplated that the magnetic circuit 11 of the present invention willfurther include additional pole pieces extending from the rectangularpole pieces 18 and 23. More specifically, a rectangular shaped straightside pole piece 26 is mated to extend from the north pole 27 in the samedirection as magnet 13. Similarly, another rectangular shaped straightside pole piece 28 is mated to extend from and in the same direction asthe south pole 29 of magnet 14. The resultant seven-piece magneticcircuit is assembled by bolts 30 extending between pole pieces on eachside of the magnets to threadingly engage the pole piece opposite thepole piece at the bolt head. Other suitable means can be employed tofacilitate holding the magnetic circuit structure together, e.g.brackets secured between pole pieces on opposite sides of each magnet orby welding the magnets and pole pieces together.

Any of the permanently magnetic materials can be employed inconstructing the magnetic circuit of the present invention. However, fora given size air gap 15 and de sired air gap magnetic field intensity,some materials are more desirable than others. from the standpoint ofmagnetic circuit size and efficiency. The selection of magnet materials,generally, is based upon the peak energy product characteristic of thematerial, i.e., the maximum external energy that can be maintained by aunit volume of the material. Furthermore, the peak energy product isgoverned, to some extent, by the residual induction and coercive forcecharacteristics of the material. In the case of the present invention,it has been found particularly advantageous to utilize magnetsconstructed from materials having a peak energy product of at least5.0)(10 gauss-oersteds. A particular material having a peak energyproduct falling in this range is heat treated cast alnico, consisting ofiron, nickel, aluminum and cobalt, cooled in a magnetic field and whosemagnetic orientation is accomplished during heat treatment. Such amaterial is manufactured by Indiana General Corporation of Valparaiso,Indiana under the designation of Alnico V and has a residual inductancecoefficient of 12.5 kilogauss, a coercive force coefficient of 600oersteds and a peak energy product of 5.25 X 10 gauss-oersteds.

For highest possible operating efficiency, i.e., maximum air gap fieldwith the minimum magnet material, the magnet structure is constructedsuch that the magnet material operates a the knee of the demagnetizationcurve of the material. For a given magnetization length, air gap size,air gap field, and magnet material, the required magnet structure can bedetermined by standard techniques. A particular magnetic circuit 11constructed to provide a 1000 gauss air gap magnetic field for asputter-type ionic vacuum pump 31 having two pump element units 32 isshown in FIGS. 1-3. As shown therein, the vacuum pump 31 comprises avacuum envelope 33 having a main chamber 34 and two spaced apartelongated rectangular parallelepiped appendage chambers 36 extendingfrom one side of the main chamber 34 defining three interconnectedvolumes. The main chamber 34 is communicated to a chamber to beevacuated (not shown) through a port 37 terminating at a flange 38adapted to be mounted in gas tight relation to the chamber to beevacuated. A flange suitable for such gas tight coupling is the ConF-latflange described in United States Patent 3,208,758 to Maurice A. Carlsonand William R. Wheeler entitled Metal Vacuum Joint, issued Sept. 28,1965. It is noted that other vacuum pump container configurations may beemployed with the magnetic circuit of the present invention. Forexample, container configurations of only one or more than two appendagechambers 56 could be employed, with appendage chambers 36 extending fromany or all sides of main chamber 34. Also, appendage cham berconfigurations other than rectangular, that is, for example, circular,could be employed. However, the rectangular appendage chambers 36facilitate most eflicient use of the magnet material since theconfiguration of the rectangular U-shaped magnetic circuit can beconformed more exactly to that of the appendage chambers 36.

A pump element unit 32 is inserted in each appendage chamber 36. Atypical pump element unit 32 includes a multi-cell anode comprised of,for example, a plurality of parallelly disposed anode cells 39,preferably circular cylinders, held together in a rectangular-like bunchby spot welds at their tangent points 40. Additional support is providedby a strap 41 tightly wrapped about the circumference of therectangular-like bunch. The anode cells 39 are mounted spaced apart andbetween cathode plates 42 and 43 with the principal axes of the cells 39perpendicular thereto, The cathode plates 42 and 43 are constructed fromreactive material that is disintegratable upon ion bombardment, e.g.,titanium. Alternatively, a coating of such material on a support membercould serve as cathode plates. Other multi-cell anode configurationsother than that formed by the individual circular cylindrical anodecells 39 could be employed, such as, a rectangular cylindrical cell orany partitioning means defining a plurality of open ended cellularcompartments. In all cases, the anode cells 39 must be open enderdcellular compartments mounted with a cathode structure covering the openends of the cellular structure.

Mounting of the anode cells 39 is accomplished by insulators 44 securedbetween strap 41 and conductive plate 46 secured to cathode plates 42and 43. The required electrostatic field is established by connecting ahigh volt age source, e.g., 3000 v. DC, (not shown) through a highvoltage feedthrough 47 connecting to the anodes 39 by conductors 48electrically connected to strap 41.

Each appendage chamber 36 is arranged to be received in nested relationby the air gap of a magnetic circuit structure 11 including elongatedrectangular plate magnets and pole pieces. Magnetic circuit 11 may alsobe mounted within the appendage chamber 36 to receive in its air gap 15pump element unit 32. The adjacent magnetic circuits 11 are mountedbetween platforms 49 (top platform shown) to have like poles proximateeach other. With like poles arranged proximate, the magnetic fieldsgenerated by each magnetic circuit 11 do not interlink to interfere withor cancel one another in the pump element unit regions. In fact, theleakage flux of each magnetic circuit 11 links with the air gap magneticfield of its adjacent magnetic circuit to produce an enhanced air gapmagnetic field. Furthermore, the magnetic fields generated by eachmagnetic circuit 11 in the space 51 therebetween are in oppositionthereby tending to cancel one another therein. The etfect of suchcancellation is a reduction of the stray magnetic field in the vicinityof the sputter-ion vacuum pump 31.

A particular magnetic circuit structure constructed to provide a 1000gauss air gap magnetic field for the sputter-ion vacuum pump 31 had thefollowing specifications. The overall dimensions of magnetic circuit 11measured 4.50 inches wide, 7.375 inches long, 14.55 inches high, anddefined an air gap 15 of 2.5 inches wide by 6.25 inches long. Magnet 12measured 2.5" x 1.126" x 14.55". Each corner pole piece 18 and 23measured 1.0" x 1.25 x 14.55". Side magnets 13 and 14 measured 1.0" x1.875" x 14.55. The side pole pieces 26 and 28 measured 4.25 inches longby 14.55 inches high. More efficient use of the side pole pieces 26 and28 was obtained by tapering the .pole pieces from a 1.0 inch maximumwidth proximate the side magnets to a inch width at their extremities.The magnets 12, 13 and 14 were constructed from the hereinbeforedescribed alnico material. The pole pieces 18, 23, 26 and 28 wereconstructed from soft iron or low reluctance mild steel. The magnitudeof the air gap magnet field can be varied by varying the length ofmagnets 12, 13 and 14.

Referring to curve (a) of FIGURE 4, the air gap magnet field intensityprofile of the above identified magnetic circuit 11 taken in thedirection of the extending arms of the U-shaped configuration along thecenter line of air gap 15 was found to be 500 gauss along the surface ofmagnet 12 facing air gap 15. The air gap magnetic field intensityincreased to 1000 gauss at the junction of side pole pieces 26 and 28and side magnets 13 and 14. The air gap magnetic field intensityremained substantially at 1000 gauss in the air gap region between thepole pieces 26 and 28 and decreased towards zero in the region beyondthe air gap 15.

Curve (b) of FIGURE 4 depicts the air gap magnetic field intensity of amagnetic circuit of the type described in the hereinbefore identifiedMeyerers Patent 3,182,234. As shown by the curve, the air gap magneticfield intensity goes to zero at the web of the U-shaped configuration.This is because the magnetic potential along the surface of the polepiece located at the web of Meyerers magnetic circuit is zero. Hence, itis seen that the air gap magnetic field-intensity profile of themagnetic circuit of the present invention is considerably enhanced overthat of Meyerers. Furthermore, for a given maximum air gap magneticfield intensity, a Meyerers type magnetic circuit configuration would bemuch larger than one constructed in accordance with the presentinvention. This is seen when it is considered that to generate amagnetic field of a given intensity, a given length of magnetization isrequired. Hence, to generate a 1000 gauss magnet field, the length ofthe side magnets in Meyerers magnetic circuit would be longer than thoseof the magnetic circuit of the present invention by one-half the lengthof the Web section of the U-shaped configuration.

Curve (c) of 'FIG. 4 depicts the air gap magnetic field intensityprofile of a U-shaped magnetic circuit configuration having a singlemagnet disposed at the web of the U and having the same overalldimensions as the magnetic circuit configuration of the presentinvention. Although the air gap magnetic field profile is improved, itis seen that the intensity is only one-half that of the magnetic circuitof the present invention. To generate a magnetic field intensitycomparable to that generated by the magnetic circuit of the presentinvention, a considerably larger single magnet type magnetic circuitstructure would be required employing a permanent magnet constructed ofmaterial whose coercive force coefficient at least equaled the desiredair gap magnetic field.

As noted hereinbefore, many advantages accrue to the magnetic circuit ofthe present invention as a result of its compactness, increasedefiiciency, and enhanced air gap magnetic field intensity profile. Mostimportantly is the ability to construct a more compact and efficientsputterion vacuum pump.

While the present invention has been hereinbefore described with respectto a single embodiment, many modifications and variations are possiblewithin the scope of the invention. For example, a U-shaped magneticcircuit configuration of more than three magnets and four pole piecescould be constructed by adding additional pole pieces and magnets inalternation in each leg of the U- shaped configuration. Also, more thanone magnet could be positioned in the web section of the U-shapedconfiguration. Furthermore, the pole pieces could be curved sectionsinstead of straight since the magnetic length is unaffected.

Therefore, the scope of the magnetic circuit of the present invention isnot intended to be limited except by the terms of the following claims.

What is claimed is:

1. An improved U-shaped magnetic circuit comprising first, second andthird straight permanent magnets arranged in a generally U-shapedmagnetic potential adding configuration with said first magnet disposedat its web and said second and third magnets at its arms, and first andsecond pole pieces, said first pole piece secured at the north pole ofsaid first magnet, said second pole piece secured at the south pole ofsaid first magnet, said second magnet secured at said first pole piecewith its south pole adjacent thereto, said third magnet secured at saidsecond pole piece with its north pole adjacent thereto.

2. The magnetic circuit according to claim 1 further comprising a firststraight pole piece secured at the north pole of said second magnet toextend therefrom in the same direction as said second magnet, and asecond straight pole piece secured at the south pole of said thirdmagnet to extend therefrom in the same direction as said third magnet.

3. The magnetic circuit according to claim 2 wherein said pole piecesand magnets are rectangular blocks.

4. The magnetic circuit according to claim 3 wherein said first andsecond straight pole pieces each have a side which is tapered from alarge end to small end thereof, the tapered side of each pole piecedefining a surface facing away from the other straight pole piece withthe large end of said first straight pole piece proximate said secondmagnet and the large end of said second straight pole piece proximatesaid third magnet. V

5. The magnetic circuit according to claim 1 wherein said magnets areconstructed from materials having a peak energy product of at least 5.0010 gauss-oersteds.

-6. The magnet circuit according to claim 5 wherein said magnets areconstructed from alnico having a residual induction coefficient of 12.5kilogauss, a coercive force coeificient of 600 oersteds and a peakenergy product of 5.25 10 gaus s-oersteds. 7

References Cited I UNITED STATES PATENTS Lloyd et a1 23069 King et a1.335306 Adler 335-304 Holland 313-7 X

